In position-measuring devices which are used in the automation sector, data is often transmitted between the position-measuring device and a downstream sequential electronics via digital, serial interfaces. In this context, on the side of the position-measuring device, an architecture is provided such that it includes a signal-generating unit which is connected to a communication unit via an internal interface unit. The communication with the sequential electronics in turn takes place via the communication unit. For example, with the aid of the signal-generating unit, analog, position-dependent signals are generated in a conventional manner from the scanning of a suitable measuring graduation and suitably conditioned in order to be transmitted in serial form via the communication unit to the sequential electronics. The internal interface unit is provided in order to be able to flexibly combine greatly differing communication units with greatly differing signal-generating units within the framework of a modular system concept.
In such an architecture, the transmission of a measurement-data request instruction from the sequential electronics to the signal-generating unit and its execution by the signal-generating unit, i.e., the actual measured-value acquisition, must be regarded as fundamentally time-critical. For example, via the measurement-data request instructions, instantaneous positional data are fetched from the position-measuring device for control purposes. To ensure high control performance on the side of the sequential electronics, it is desirable to acquire measurement data, or more precisely, positional data, in a manner as free of delay as possible.
European Published Patent Application No. 0 660 209 describes a position-measuring device to which a processing unit sends commands in the form of data words transmitted in bit-serial fashion, and as a function of these commands, measured position values or parameters are requested from the position-measuring device, or parameters are sent to the position-measuring device.
Furthermore, U.S. patent application Publication No. 2001/0001540 describes a position-measuring device in which, in response to a positional-data request, a measured position value is sent which is based on a measured position value that was already measured at an earlier point in time and mathematically corrected such that it corresponds to the instantaneous position value at the moment of the positional-data request. Disadvantageous in such a position-measuring device is that the accuracy of the measured position values depends upon the accuracy of the parameters needed for the correction, such as the time span between the acquisition of the measured position value and the arrival of a positional-data request, as well as the rate of change of the measured position values (e.g., traversing speed of a machine-tool axis monitored by the position-measuring device).